A tire having a radial carcass reinforcement comprises, in a known manner, a tread, two inextensible beads, two sidewalls joining the beads to the tread and a crown reinforcement or “belt” based circumferentially between the carcass reinforcement and the tread, this belt consisting of various plies or layers of rubber which may or may not contain reinforcement elements (“reinforcing threads”) such as cords or monofilaments, of the metal or textile type.
Said tire belt generally consists of at least two superposed belt plies or layers, usually termed “working” plies or “crossed” plies, the generally metallic reinforcing cords of which are in practice arranged parallel to one another within a layer, but crossed from one layer to the other (i.e. inclined, whether symmetrically or not, to the median circumferential plane), via an angle which is generally between 10° and 45° depending on the type of tire in question. These crossed layers or plies may be finished off by various other auxiliary rubber plies or layers which may or may not contain reinforcements.
In particular, as is well known, the belt may for example comprise one or more crown plies or layers, called “protective” plies, located beneath the tread and surmounting the working crown ply or plies, the role of which protective plies is to protect the rest of the belt from external attacks, tearings or other perforations. This is for example the general case in the belts of tires for heavy goods vehicles or civil engineering vehicles.
These protective plies or layers must be sufficiently flexible and deformable so as, on the one hand, to follow as closely as possible the shape of the obstacle on which the belt bears during rolling and, on the other hand, to prevent the penetration of foreign bodies radially towards the inside of said belt. To meet such criteria requires, in a known manner, the use in these protective layers of reinforcements or cords having a high elasticity and a high breaking energy.
Steel cords called “strand cords” (also termed “HE” (high elongation) cords) are normally used, these being assembled by the known technique of stranding and consisting of a plurality of metal strands twisted together in a helix, each strand comprising several (three or more) steel wires which are also wound together in a helix.
Such elastic strand cords have been described in a large number of patents or patent applications (see for example, U.S. Pat. No. 3,686,855, U.S. Pat. No. 4,176,705, WO 2004/003287 or US 2005/0183808), in particular to reinforce protective crown plies of tires for industrial vehicles such as heavy or civil engineering vehicles (see for example U.S. Pat. No. 5,843,583, U.S. Pat. No. 6,475,636, WO 2004/003287 or US 2005/0183808, WO 2004/033789 or U.S. Pat. No. 7,089,726, WO 2005/014925 or US 2006/0179813).
A well-known drawback of these strand cords is that they are expensive, this being so on two counts: firstly, they are prepared in two steps, namely by the prior manufacture of the strands followed by assembly by twisting these strands, and, on the other, they generally require their wires to have a high twist (i.e. a very short helix pitch), this twist being essential in order to give them the desired elasticity, but this involves low manufacturing rates. This drawback of course has repercussions on the cost of the tires themselves.
Another general drawback of these elastic strand cords is their large size (outside diameter). Now, a major objective of a tire manufacturer wishing to reduce the rolling resistance of said tires, and therefore the hysteresis, among others, of tire belts, is at the present time to reduce the thickness of the composite reinforcing fabrics used in these belts, in particular by developing cords that are more compact.